In order to form a high-definition image in an electrophotographic image forming apparatus using a photosensitive member, it is necessary to control the apparatus with measuring electric potential of a charged photosensitive member. For measuring an electric potential of a photosensitive member, there is a method of performing measurement by providing a detecting electrode near the photosensitive member so as not to contact the photosensitive member, changing capacitance between the detecting electrode and photosensitive member to change periodically lines of electric force which are incident into the detecting electrode from the photosensitive member, and measuring a slight amount of charges induced on the detecting electrode by electrostatic induction. As an example of a noncontact electric potential measuring apparatus performing measurement by this method, there is an article of changing capacitance between a detecting electrode and a photosensitive member using mechanical constitution. FIG. 18 shows a conceptual structural diagram of such a noncontact electric potential measuring apparatus. In this diagram, VD denotes a surface potential of a object to be measured 501, reference numeral 102 denotes charge detection means, and VOUT denotes a signal outputted from the charge detection means 102. For changing mechanically capacitance C1 between a detecting electrode 105 and the photosensitive member 501 which is a object to be measured, there are a method of installing a part, which changes periodically lines of electric force which are incident into a detecting electrode, between the photosensitive member 501 and detecting electrode 105, and a method of moving the detecting electrode 105 periodically. U.S. Pat. No. 4,720,682 discloses the constitution of inserting a fork-shaped shutter between a photosensitive member and a detecting electrode, interrupting periodically lines of electric force which reaches the detecting electrode from, the photosensitive member by moving periodically the shutter in a direction parallel to a surface of the photosensitive member, consequently, changing an effectual area of the detecting electrode in view of a measuring plane to change capacitance between the photosensitive member and detecting electrode.
U.S. Pat. No. 3,852,667 discloses the constitution of locating a metal shield material, having an aperture portion, in a position opposite to a object to be measured, providing a detecting electrode at a tip of a fork-shaped vibration element, changing the number of lines of electric force which reach the detecting electrode by moving a position of the detecting electrode in parallel directly under the aperture portion, and hence, changes capacitance.
U.S. Pat. No. 4,763,078 discloses the constitution of locating a detecting electrode at a tip of a cantilever-like vibrator, changing periodically a distance between a object to be measured and the detecting electrode by vibrating the vibrator, and hence, changing capacitance.
However, there are tasks described below in the above-mentioned conventional examples. A schematic diagram of an electric potential measuring apparatus for explaining this task is shown in FIG. 17. In the electric potential measuring apparatus, an actuator 108 which generates mechanical vibration for changing the capacitance C1, and the detecting electrode 105 which measures electric charges induced are located. A portion to which a drive signal VDRIVE of an actuator is applied forms parasitic capacitance C2 with the detecting electrode, and charges caused by it (hereafter, this is called “parasitic charges”) is generated on the detecting electrode. As a result, the drive signal VDRIVE to the actuator superposes the detection signal VOUT from the detecting electrode which is obtained with corresponding to a surface potential VD of the photosensitive member 501. Since the detection signal VOUT is extremely minute, there is a possibility that detection power may drop as a result of being superposed.